Processing of fibrous monolithic ceramics

grantor: University of TorontoThis work examined the processing of ceramic composites to achieve a controlled microstructure with a fibrous morphology, referred to as fibrous monolithic ceramics. Such controlled microstructures offer the possibility of enhanced performance, such as improved fracture behavior. The processing steps included: powder/binder selection, powder/binder mixing, feedstock extrusion, fiber coating, molding of fibers, debinding via pyrolysis and sintering. Materials produced using this technique included nickel-zirconia and graphite-alumina composites. Load vs displacement curves of samples of these composites in three- and four-point bending show a graceful step-wise failure. The fibrous nature of these samples improved their overall toughness and resistance to cracking primarily through crack deflection between fibers. The binder removal or pyrolysis stage was the processing stage where most defects occurred. An experimental debinding map was constructed for 60-vol% alumina-polypropylene feedstock samples, which defined heating rate-temperature conditions for sample bloating. The significance of certain process variables for debinding was examined via statistical experimental design. The variables found to be statistically significant for constant temperature pyrolysis of compacts included: powder type, debinding atmosphere, binder composition, debinding temperature and specimen geometry. A response surface describing the effects of these variables on the debinding time was developed. Based on the experimental debinding characterization, a mathematical model was developed to predict the incidence of bloating in powder injection molded compacts as a function of sample geometry, debinding heating rate or debinding temperature, ceramic volume fraction and binder wax addition fraction. Transport coefficients were determined via iterative procedures fitting experimental weight loss data to model-predicted weight loss for a variety of debinding conditions. The model used a finite difference scheme, allowing for a concentration dependence of the diffusion coefficient; however, it was observed, that for the present case, this dependency was negligible. Model-predicted weight loss and bloating conditions for a variety of sample geometries and compositions under a variety of debinding conditions showed good agreement with experimental observations.Ph.D